Back to the Future: Next-Generation Vacuum ElectronicsMillimeter wave vacuum tubes, including ones like the travelling wave tube (TWT) depicted here, amplify signals by exchanging kinetic energy in the electron beam (shown as a blue line) with electromagnetic energy (shown as a wave) in the signal.
Photo courtesy of DARPA

With its new Innovative Vacuum Electronic Science and Technology (INVEST) program, DARPA aims to develop the science and technology base for new generations of more capable VEDs.

Solid-state electronics began to overtake vacuum tubes in radios, computers, and other electronic and radio frequency gadgetry more than 60 years ago. But now it’s the Silicon Age. Even so, vacuum electronic devices, whose origins date to the 19th century, are still present in everyday life.

Microwaves of today come from a magnetron, the vacuum tube that made radar possible in the first half of the 20th century. And traveling wave tubes (TWTs), not solid-state amplifiers, generate the strong electromagnetic signals in communication satellites because of their exceptional on-orbit reliability and high power efficiency.

It’s the unique ability of vacuum tube electronic devices to generate high-frequency signals at chip-melting operating powers that makes possible modern aviation radar systems for navigation and collision avoidance. There are also more than 200,000 vacuum electronic devices (VEDs) now in service in the Department of Defense, powering critical communications and radar systems that cover the land, sea, air, and space.

“Any time you need to operate at the outer reaches of the power-frequency parameter space, vacuum tubes are the technology of choice,” said Dev Palmer, program manager for INVEST in DARPA’s Microsystems Technology Office (MTO), in a press release. “But at the high millimeter-wave frequencies of interest to this program, the design and construction of VEDs is an intricate, labor-intensive process that requires exquisite modeling tools, exotic materials, and expensive, high-precision machining.”

Physical scaling laws have been the showstopper for millimeter-wave VEDs so far. As engineers push the operating frequency of electronic devices upward, the output power from the same devices goes down. With INVEST, Palmer aims over the next four years to create a community of researchers that will find ways through this technical bottleneck.

Notwithstanding the popular notion that vacuum electronics are old-fashioned, the incentive to overcome technical and cost barriers to obtain next-generation VEDs is only getting stronger.

“The worldwide availability and proliferation of inexpensive, high-power commercial amplifiers and sources has made the electromagnetic spectrum crowded and contested in the radio frequency (RF) and microwave regions,” according to MTO’s just-published Broad Agency Announcement (BAA), which invites the technical community to submit proposals for research that would take VED technology to new heights of power and frequency.

VEDs are capable of operating at higher frequencies and shorter wavelengths (in the millimeter wave region) and can outperform the current generation of devices. This will provide significant defense advantages. Higher power operation yields RF signals that are “louder” and thereby harder to jam and otherwise interfere with. Meanwhile, higher frequency operation brings with it vast swaths of previously unavailable spectrum. This too opens the way to more versatile communication, data transmission, and other capabilities that will be beneficial in both military and civilian settings.

To open pathways towards those advances, the INVEST program aims to strengthen the science and technology base for new generations of vacuum tubes operating at millimeter-wave frequencies above 75 GHz.

Those awarded contracts under the program, will take on fundamental research projects in areas that include physics-based modeling and simulation of VEDs, innovative component design, electron emission processes, and advanced manufacturing.

“As you push frequencies up, you can’t use conventional manufacturing techniques anymore,” Palmer said in the release. Palmer emphasized the tiny size and ultraprecise alignment of millimeter-wave VED components, among them high-current-density cathodes, tiny vacuum envelopes, and micro-parts that extract the RF signals amplified inside the component.

“If you could print the whole structure with a 3-D printer, so that everything was aligned right off the assembly line, it would make it much easier,” Palmer explained in the release.

The ultimate and most welcome outcome would be to transform the new scientific understanding and engineering know-how that emerges from the INVEST program into novel tools for analyzing, synthesizing, and optimizing new VED designs and then deploying innovative advanced manufacturing methods, including 3-D printing, to actually produce the devices, Palmer said, adding that it’s a “beautiful vision.”